Dual coil variable reluctance wheel speed sensor

ABSTRACT

A wheel speed sensor system is disclosed which produces an output directly at the sensor. The sensor utilizes two separate coils that are wound out of phase from each other and exhibit a relatively high signal-to-noise ratio. Also disclosed is an anti-skid braking system that utilizes the wheel speed sensor system.

FIELD OF THE INVENTION

[0001] The present invention relates to a sensor assembly for sensingthe angular velocity of a rotating body. In particular, the inventionrelates to a sensor assembly for determining the angular velocity of avehicle wheel. The present invention also relates to an anti-skid systemutilizing an improved sensor assembly for sensing the angular velocityof a wheel controlled by the anti-skid system. Although the inventionmay be used in a variety of applications, it is particularly adapted formeasuring vehicle wheel speed.

BACKGROUND OF THE INVENTION

[0002] Inductive magnetic sensors are commonly used for automotiveapplications and the like to provide timing signals which enable thedetermination of position and speed of a rotating wheel. For example,specific applications may include the determination of engine crankshaftposition and speed (i.e., RPM) and the determination of wheel speed foranti-lock braking systems. Inductive magnetic sensors generally used forthese types of applications are commonly referred to as variablereluctance sensors.

[0003] The variable reluctance sensor is generally located adjacent to arotating wheel which typically has a plurality of circumferentiallyspaced slots formed therein. The sensor has an inductive magneticpick-up that generally comprises a pick-up coil wound on a core composedof magnetic or ferrous material. As the wheel rotates relative to thepick-up coil, an alternating voltage is generated in the pick-up coilwhen the slots on the wheel travel past the sensor. The alternatingvoltage must then be correctly decoded to recognize periodic high orpositive voltage levels. The frequency of the alternating voltage isthen determined to obtain rotational speed information about the wheel.

[0004] The more turns the coil is wound on a core, the larger thepeak-to-peak voltage will be in the output of the coils in any variablereluctance sensor. A coil needs to have more turns to create a highvoltage in order to send an undistorted signal to an electronic controlunit. However, the higher the voltage, the higher the temperature of thecoils. In order to wind the coils with more turns around a core, a wirewith a small diameter is required.

[0005] Examples of prior wheel speed sensing systems are shown in U.S.Pat. Nos. 3,854,556; 3,938,112; 3,961,215; 3,988,624; and 4,029,180, allof which are hereby incorporated by reference. Typically, such priorsystems utilize a ferromagnetic rotor rotatable with a vehicle wheel anda sensing device opposing the rotor across an air gap and fixed againstrotation, such as to an axle housing of the vehicle. The air gap may beaxial or radial. The sensing device is typically of an electromagnetictype with an output signal of frequency proportional to the angularvelocity of the rotor. In sensors of this general type, a continuingproblem has been the presence of false information or noise in theoutput signal of the sensing device due to variations in the size of theair gap during operation. These variations are often due, for example,to rotor vibration or runout in the direction of the air gap. Such falseinformation or noise in the output signal may cause production ofimproper lock signals in an anti-lock system.

[0006] Past attempts to overcome this problem have included, forexample, manufacture of components to close tolerances or elaboratesensor mounting techniques. However, these approaches have been costlyand unsatisfactory.

[0007] Another prior approach to the problem is disclosed in thepreviously noted U.S. Pat. No. 3,854,556 in which a rotor and statorassembly are mounted alongside a vehicle wheel such that the rotorrotates with the wheel and in close proximity to the stationary stator.The stator utilizes a particular configuration of coil windings andarrangement of magnetic elements which are said to eliminate noise orinaccurate sensor readings resulting from changes in the air gapdistance. The assembly utilizes two separate coils that further increasethe complexity of the system. Moreover, the structure and manufacture ofthat sensor system is relatively costly to produce, install, andmaintain.

[0008] Accordingly, an object of the present invention is to provide awheel speed sensor assembly having a high signal to noise ratio and thatis relatively simple and inexpensive to produce.

[0009] Furthermore, many applications for wheel sensing devices involveexposure to temperatures as high as 180° C. Although some sensingdevices may be able to withstand such high temperatures, it is desirableto provide a relatively simple and robust sensor that can withstandrepeated and prolonged exposure to high temperatures.

[0010] Vehicle anti-skid systems typically employ a wheel speed sensorassociated with each vehicle wheel being controlled. Each sensorprovides a signal proportional to the angular velocity of its associatedwheel. Each of these signals is utilized by anti-skid circuitry which,in dependence upon the signal value and derivatives thereof and perhapsthat of other signals, provides a skid signal. This skid signal is thenutilized to regulate the braking forces applied to one or morecontrolled wheels. Since the provision of the skid signal depends uponthe sensed angular velocity of one or more wheels, it is exceedinglyimportant that the sensor assembly provides a frequency signal whichexhibits a high degree of accuracy. In view of the concern for retaininga high level of accuracy in the signal, it is undesirable to furthersubject the signal to numerous conversions or filtering operations.

[0011] Accordingly, it is a further object of the present invention toprovide a wheel speed sensor assembly that is accurate and may readilybe incorporated into an anti-skid braking system. That is, it would beparticularly beneficial to provide a wheel speed sensor assembly thatprovided an accurate digital output directly from the sensor assembly.

[0012] The present invention meets these and other objects as more fullydescribed herein.

SUMMARY OF THE INVENTION

[0013] In a first aspect, the present invention provides a vehicle wheelspeed sensor system adapted for providing a control signal indicative ofthe angular velocity of a vehicle wheel. This system includes a firstcoil secured or otherwise affixed to a vehicle and positioned adjacentto a wheel of the vehicle. The first coil provides a first outputsignal. The system also includes a second coil secured to the vehicleand positioned adjacent to the same wheel of the vehicle, and the secondcoil providing a second output. The first and second coils are wound outof phase with respect to each other. The speed sensor system further hasa circuit having two inputs, each in communication with the first andsecond outputs of the noted first and second coils. The circuit providesa control signal indicative of the angular velocity of the vehiclewheel.

[0014] In another aspect, the present invention provides a vehicleanti-skid braking system. The system includes a collection of wheelspeed sensor systems, a collection of braking assemblies, and anelectronic control unit in communication with the wheel speed sensorsystems and braking assemblies. Each of the wheel speed sensor systemsis positioned adjacent to a vehicle wheel and provides a control signaloutput that is indicative of the angular velocity of the wheel. Each ofthe braking assemblies is also positioned adjacent to a correspondingwheel and is adapted for receiving a control signal for selectivelyapplying braking force to that wheel. The electronic control unit is incommunication with the collection of wheel speed sensor systems and thecollection of braking assemblies and provides one or more controlsignals for the collection of brake assemblies. Each control signal istransmitted by the electronic control unit to a corresponding brakingassembly for selectively applying braking force to each of the wheels ofthe vehicle. These control signals are based, at least in part, upon thesignal control outputs from the collection of wheel speed sensorsystems. Each of the wheel speed sensor systems includes a first coiladjacent to the vehicle wheel and a second coil adjacent to the vehiclewheel. The two coils are out of phase with respect to each other.

[0015] In yet another aspect, the present invention provides a vehicleanti-skid braking system for controlling braking of a vehicle. Thissystem includes a first coil and a second coil secured to the vehicleand positioned in close proximity to a wheel of the vehicle. The twocoils provide first and second control signals that are proportional tothe angular velocity of the wheel. The first control signal is in theform of a sine wave upon rotation of the wheel. The second controlsignal is in the form of a cosine wave upon rotation of the wheel. Thevehicle anti-skid braking system also includes a circuit incommunication with the first and the second control signals forconverting those signals to an output frequency signal which isproportional to the angular velocity of the wheel and in the form of asquare wave upon rotation of the wheel. The vehicle anti-skid brakingsystem additionally includes an electronic control unit having an inputfor receiving the output signal from the circuit and an output fortransmitting a braking control signal. The input of the electroniccontrol unit is in communication with the output signal of the circuit.The vehicle anti-skid braking system has a braking assembly associatedwith each of the wheels of the vehicle. The braking assembly has aninput for receiving a braking control signal from the output of theelectronic control unit and is adapted to apply a braking force to itscorresponding wheel based upon the braking control signal. The input ofthe braking assembly is in communication with the output of theelectronic control unit.

[0016] These and other aspects of the present invention are describedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic of a preferred embodiment wheel speed sensorsystem in accordance with the present invention; and

[0018]FIG. 2 is a schematic of a preferred embodiment anti-skid brakingsystem utilizing the preferred wheel speed sensor system in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Generally, the present invention provides a wheel velocity sensorcomprising two separate coils that are wound out of phase and apartmagnetically around a core, and a back-biasing magnetic disk. Duringoperation of the preferred embodiment sensor, described herein, when thesensor detects rotation of a corresponding wheel, the two coils producetwo waveforms, each out of phase with respect to the other, e.g. a sineoutput signal and a cosine output signal. These two outputs arepreferably connected to an R/S flip-flop circuit, but not limited to aparticular electronic circuitry, such as may be provided within orexternal to the sensor assembly that provides an output signal. Incertain preferred embodiments, the output signal is a square waveoutput. The output signal is then directly transmitted to an anti-lockbrake system (ABS) electronic control unit (ECU) and accepted as thewheel speed sensor output. With additional circuitry, the power producedby the variable reluctance coils may be used as power to the electronicsin the sensor assembly. The circuitry that interprets the analog outputof the coils is next to the coils which does not require a large peak topeak voltage. As noted herein, because the coils utilize fewer turnsthan in comparative sensors, a more robust sensor is provided, thusimproving the temperature and service conditions for the device. Sincefewer turns are used, a larger diameter wire is used with a thicker wireinsulation, thereby allowing the coil of the wire to survive throughhigher temperatures.

[0020] Preferably, the two separate coils are wound on the same core andshare a common axis. The present invention includes numerous windingcombinations of the two coils. For instance, a first and a second coilmay be wound in essentially the same configuration on a common core.Alternately, the two coils may be wound along separate regions of acommon core. And, the two coils may be wound along separate regions of acommon core and overlap one another or at least share a region of thecommon core. The two coils may be wound in nearly any pattern orarrangement on the common core.

[0021]FIG. 1 illustrates a schematic of a preferred embodiment wheelspeed sensor system 100. This system 100 comprises a first coil 110 anda second coil 120. Each of the coils is stationary and mounted orotherwise affixed to the vehicle. As noted, it is preferred that the twocoils are wound upon a common core. The coils are positioned alongsidethe wheel whose angular velocity is to be monitored or measured. Thecoils 110 and 120 are configured from about 45° to about 135° out ofphase with respect to each other and preferably about 90° out of phase.That is, the coils are positioned with respect to each other and withrespect to the wheel such that they are out of phase with each other.The coil 110, upon sensing wheel rotation, produces an output 112 thatcorresponds to a first waveform and preferably a sine wave. The coil120, upon sensing wheel rotation, produces an output 122 thatcorresponds to a second waveform out of phase with respect to said firstwaveform and preferably a cosine wave. The system further comprises anR/S flip-flop circuit 130 as known in the art. The outputs 112 and 122are directed to the R/S flip-flop circuit 130. As will be appreciated,the circuit 130 is connected to a power source 132 and a ground 134.Upon receiving the coil outputs 112 and 122, the circuit 130 produces anoutput signal, which may be in the form of a square wave output signal190. The square-wave signal 190 is directed to an anti-lock brake system(ABS) electronic control unit (ECU) 19.

[0022] It is contemplated that with additional circuitry, the powerproduced by the variable reluctance coils, i.e. coils 110 and 120 inFIG. 1, may be used as power to other electronics in the wheel speedsensor. Furthermore, if the wheel speed sensor is self-powered, then theECU need only supply a relatively small voltage bias, e.g. 2.5 volts, atlower speeds to the wheel speed sensor.

[0023] It will be understood that the present invention is not limitedto particular electronic circuits such as the noted R/S flip flop forreceiving the outputs from the coils. It is envisioned that numeroussignal processing elements could be used to receive the coil outputs,for example, an amplifier, an amplifier and filter, an analog to digitalconverter, such a converter with a Schmitt Trigger, and smartelectronics such as based upon a microprocessor. Moreover, it iscontemplated that depending upon the system, the electronic circuit atthe sensor level could be eliminated and the coil outputs sent directlyto an ECU. Accordingly, the output from the sensor, after appropriateprocessing, if necessary, may exhibit a wide array of waveforms. Thepresent invention is not limited to the sensor output corresponding to asquare wave output.

[0024]FIG. 2 illustrates a preferred embodiment anti-skid braking systemutilizing the preferred embodiment wheel speed sensor system inaccordance with the present invention. FIG. 2 illustrates a preferredanti-skid braking system 300 comprising a plurality of preferred wheelspeed sensors 100 a, 100 b, 100 c, and 100 d. The system 300 depicted inFIG. 2 is used with four (4) wheels 210 a, 210 b, 210 c, and 210 d. Thepresent invention anti-skid braking system can readily be used inconjunction with a lesser or greater number of wheels. The preferredembodiment anti-skid braking system 300 further comprises a plurality ofbraking assemblies 200 a, 200 b, 200 c, and 200 d. As will beappreciated, each wheel has a corresponding sensor and braking assemblyassociated with the wheel. Thus, wheel 210 a has sensor 100 a andbraking assembly 200 a associated with it. Wheel 210 b has sensor 100 band braking assembly 200 b associated with it. Wheel 210 c has sensor100 c and braking assembly 200 c associated with it. And wheel 210 d hassensor 100 d and braking assembly 200 d associated with it.

[0025] Each of the respective sensors 100 a-d and braking assemblies 200a-d are in electrical communication with the anti-lock brake system(ABS) electronic control unit (ECU) 195 described in FIG. 1.Corresponding electrical conductors 220 a, 222 a, 220 b, 222 b, 220 c,222 c, 220 d, and 222 d are utilized to provide electrical signalcommunication between the sensors and braking assemblies and the ECU. Aswill be appreciated, the ECU 195 controls the braking assemblies 200a-d, at least in part, any information received-from the sensors 100a-d.

[0026] It will be appreciated that the square wave signal output 190(FIG. 1) from each of the sensors 100 a-d, is directly used by the ECU195. And, it will be understood that each of the sensors 100 a-dincludes two coils (such as coils 110 and 120 in FIG. 1) that providerespective waveforms, for instance sine and cosine outputs. Each pair ofcoils is illustrated in FIG. 2 as 110 a-d and 120 a-d. Each pair ofwaveforms, e.g. sine and cosine outputs, is directly utilized by arespective sensor 100 a-d at a corresponding wheel 210 a-d to provide ahighly accurate square wave output that is transmitted to the ECU 195 bya corresponding conductor 220 a-d. The ECU 195 in turn, transmits outputsignals via conductors 222 a-d to control each of the respective brakingassemblies 200 a-d.

[0027] The preferred embodiment wheel speed sensor system is believed toprovide a significant improvement over currently known relativelysensitive sensor designs. That is, the preferred embodiment sensorsystem is relatively simple and utilizes coils having fewer windings orturns as compared to comparable sensors. Accordingly, a more robustmagnetic wire may be used, thereby increasing the high temperatureoperating limits of the sensor.

[0028] The foregoing description is, at present, considered to bepreferred embodiments of the present invention. However, it iscontemplated that various changes and modifications apparent to thoseskilled in the art may be made without departing from the presentinvention. Therefore, the foregoing description is intended to cover allsuch changes and modifications encompassed within the spirit and scopeof the present invention, including all equivalent aspects.

We claim:
 1. A vehicle wheel speed sensor system adapted for providing acontrol signal indicative of the angular velocity of a vehicle wheel,said system comprising: a first coil affixed to a vehicle and disposedadjacent to a wheel of said vehicle, said first coil providing a firstoutput; a second coil affixed to said vehicle and disposed adjacent tosaid wheel of said vehicle, said second coil providing a second output,the first and second coils being out of phase with respect to eachother; and a circuit having a first input in communication with saidfirst output of said first coil, a second input in communication withsaid second output of said second coil, and said circuit providing saidcontrol signal indicative of the angular velocity of said vehicle wheel.2. The vehicle wheel speed sensor of claim 1 wherein said first andsecond coils are 90° out of phase.
 3. The vehicle wheel speed sensor ofclaim 1 wherein said first output corresponds to a first waveform. 4.The vehicle wheel speed sensor of claim 3 wherein said second outputcorresponds to a second waveform out of phase from said first waveform.5. The vehicle wheel speed sensor of claim 1 wherein said control signalprovided by said circuit corresponds to a square wave.
 6. The vehiclewheel speed sensor of claim 1 wherein said first output corresponds to asine wave, said second output corresponds to a cosine wave, and saidcontrol signal provided by said circuit corresponds to a square wave. 7.The vehicle wheel speed sensor of claim 1 wherein said first and secondcoils are from about 45° to about 135° out of phase.
 8. The vehiclewheel speed sensor of claim 7 wherein said circuit is an R/S flip-flopcircuit.
 9. A vehicle anti-skid braking system adapted for controllingbraking of a vehicle having a plurality of wheels, said systemcomprising: a plurality of wheel speed sensor systems, each of saidsensor systems disposed proximate to a wheel and providing a controloutput indicative of the angular velocity of said wheel; a plurality ofbraking assemblies, each of said braking assemblies disposed proximateto a wheel and adapted for receiving a control signal for selectivelyapplying braking force to said wheel; and an electronic control unit incommunication with said plurality of wheel speed sensor systems and saidplurality of braking assemblies, and providing a plurality of controlsignals for said plurality of brake assemblies, each said control signaltransmitted to a corresponding braking assembly for selectively applyingbraking force to said wheel, said plurality of control signals based atleast in part upon said control outputs from said plurality of wheelspeed sensor systems, wherein each of said wheel speed sensor systemsincludes (i) a first coil adjacent to said vehicle wheel, and (ii) asecond coil adjacent to said vehicle wheel, said first and second coilsbeing out of phase with respect to each other.
 10. The vehicle anti-skidbraking system of claim 9 wherein said first and second coils are 90°out of phase.
 11. The vehicle anti-skid braking system of claim 9wherein said control output of each of said wheel speed sensors is asquare wave.
 12. The vehicle anti-skid braking system of claim 9 whereinfor each of said wheel speed sensor systems, said first coil provides afirst coil output corresponding to a sine wave upon rotation of saidwheel disposed proximate to said sensor system, and said second coilprovides a second coil output corresponding to a cosine wave uponrotation of said wheel disposed proximate to said sensor system.
 13. Thevehicle anti-skid braking system of claim 12 wherein said control outputof said sensor system corresponds to a square wave.
 14. The vehicleanti-skid braking system of claim 9 wherein said first and second coilsare from about 45° to about 135° out of phase.
 15. A vehicle anti-skidbraking system for controlling braking of a vehicle, said systemcomprising: a first coil secured to said vehicle and positioned in closeproximity to a wheel of said vehicle, said first coil providing a firstcontrol signal proportional to the angular velocity of said wheel and inthe form of a first waveform upon rotation of said wheel; a second coilsecured to said vehicle and positioned in close proximity to said wheelof said vehicle, said second coil providing a second control signalproportional to the angular velocity of said wheel and in the form of asecond waveform out of phase with respect to said first waveform uponrotation of said wheel; a circuit in communication with said first andsaid second control signals for converting said first and said secondcontrol signals to an output signal proportional to the angular velocityof said wheel and in the form of a square wave upon rotation of saidwheel; an electronic control unit having an input for receiving saidoutput signal from said circuit and an output for transmitting a brakingcontrol signal, said input of said electronic control unit being incommunication with said output signal of said circuit; and a brakingassembly associated with said wheel of said vehicle, said brakingassembly having an input for receiving said braking control signal fromsaid output of said electronic control unit, said braking assemblyadapted to apply a braking force to said wheel based upon said brakingcontrol signal, and said input of said braking assembly being incommunication with said output of said electronic control unit providingsaid braking control signal.
 16. The vehicle anti-skid braking system ofclaim 15 further comprising: a third coil secured to said vehicle andpositioned in close proximity to a second wheel of said vehicle, saidthird coil providing a third control signal proportional to the angularvelocity of said second wheel and in the form of a third waveform uponrotation of said second wheel; a fourth coil secured to said vehicle andpositioned in close proximity to said second wheel, said fourth coilproviding a fourth control signal proportional to the angular velocityof said second wheel and in the form of a fourth waveform out of phasewith respect to said third waveform upon rotation of said second wheel;a second circuit in communication with said third and fourth controlsignals for converting said third and fourth control signals to a secondoutput signal proportional to the angular velocity of said second wheeland in the form of a square wave upon rotation of said second wheel;said electronic control unit further having a second input for receivingsaid second output signal from said second circuit and a second outputfor transmitting a second braking control signal; and a second brakingassembly associated with said second wheel of said vehicle, said secondbraking assembly having an input for receiving said second brakingcontrol signal, said braking assembly adapted to apply a braking forceto said second wheel upon receiving said second braking control signal.17. A wheel speed sensor for providing a control signal comprising: afirst coil wound around a core; and a second coil wound separately fromsaid first coil around the core, wherein said first coil and said secondcoil are out of phase with respect to each other.
 18. The wheel speedsensor of claim 17 wherein said first and said second coils are fromabout 45° to 135° out of phase.
 19. The wheel speed sensor of claim 17wherein said first and said second coils are 90° out of phase.